The constituent elements of the structure of a vehicle, and of an aircraft in particular, must be tested to check that flaws that are inherent to the process of their manufacture, and that would be liable, in particular, to decrease the structural withstand properties of these parts in service, are absent therefrom. The size of these flaws is, generally, several orders of magnitude smaller than the size of the parts thus tested. More particularly, in the case of composite parts comprising a resin reinforced by a fibrous reinforcement taking the form of continuous fibers, these parts form integral subassemblies of the structure, which are unitarily formed by molding, and which may have complex shapes.
In the prior art, it is known to test this type of part with techniques that use the propagation of ultrasound through the thickness of the part to detect therein cracks, delamination, resin deficiencies, etc. These ultrasonic testing techniques advantageously use one or more laser beams, which are focused on the surface of the part and moved over this surface by a robot or a manipulator, to create an excitation and to obtain an interferometric measurement. These nondestructive testing techniques allow automatic “dry” testing of the entirety of the volume of the part to be performed, no acoustic coupling agent being used between the sensors and the surface on which the testing is carried out.
However, in the case of a part formed from a composite material reinforced by long or continuous fibers, flaws that are liable to have an influence on the mechanical properties of the part, but the presence of which is not detected by ultrasonic testing, or which require, in order to be detected by such testing, operating conditions that are not compatible with industrial conditions, or indeed the detection and above all characterization of which requires destructive testing, exist. Thus, flaws such as pleating or corrugation of the fibers can only be detected in micrographic sections or by nonautomatable visual methods such as the detection of an orangey “flash” when the tested zone of the part is illuminated with a white light. The visual method referred to as the “resin flash” method takes advantage of the fact that fiber pleats or corrugations create bodies of resin, mainly on the surface, in the zones in which these pleats are observed, i.e. zones in which the fibers must follow tortuous paths and in which the course of the path does not allow a tension to be applied to the fibers, such as in the concave zones of the fillet radii between two faces. These bodies of resin are detected by illuminating the surface, the light of this illumination being modified by the translucent resin that, in the absence of local reinforcement, generates a bright spot. This detection technique is as old as the use of composites comprising fibrous reinforcements in an organic matrix, to the point that the flaw and its detection technique are not often differentiated, both commonly being designated by the term “resin flash”.
Now, analysis of micrographic sections only provides local information that is not representative of the extent, and therefore the severity, of the flaw, and the flash method does not enable quantitative characterization of the flaw, the latter method in addition being highly subjective and depending on the experience of the operator implementing it.